(1) Field of the Invention
The invention relates to a method of, and an apparatus for, machining a workpiece with an end mill, a ball-end mill, a drill, a milling cutter, or another rotating cutting tool.
It is noted that "coolant" means, in this specification, fluid having the effect of removing heat and chips generated during the cutting work, such as cutting liquid or compressed air.
Furthermore, in this specification, the cutting tool has a shank and a cutting part connected thereto. The free end of cutting part forms a main cutting part which substantially performs a cutting action. And a secondary cutting part is defined as the remaining portion of the cutting part other than the main cutting part. The heat and chips are generated at a cutting point which is a portion of the workpiece being cut by the main cutting part of the cutting tool.
(2) Description of the Related Art
A cutting tool generally has higher hardness and rigidness than the workpiece to be machined. In cutting work, a rotating cutting tool is applied to the workpiece with a main cutting part thereof being forced onto a workpiece to be machined, whereby the cutting edge is advanced into the workpiece and chips are generated by the shearing action between the cutting edge and the workpiece. The rotating cutting tool is fed along a tool travel path relative to the workpiece so that the workpiece is machined into the desired shape. At this time, heat is generated because of the shearing work and the friction between the chips and the cutting tool. The heat generated during the cutting work will be transferred to the main cutting part of the cutting tool and reduce the tool life thereof. Furthermore, the heat will make a so-called built-up edge which increases the roughness of the machined surface.
These problems also become remarkable in the case of machining a workpiece of a material having low machinability, that is, a material of high hardness and/or high toughness, for example, titanium, Alloy 600 or hardened steel. These materials are not machinable by the prior art except by grinding or electrical discharge machining.
Thus, to avoid these problems, firstly, it is necessary to apply high-pressure (dynamic pressure) coolant to the shearing portion of the workpiece where the heat and the chips are generated, and to the friction portion between the chips and the face of the cutting tool to remove the heat and the chips generated during the cutting work. In other words, it is necessary to apply high-pressure coolant having sufficient pressure and flow rate to the main cutting part and the cutting point to remove the heat and the chips.
There are two known method to supply the coolant to the cutting edge. One is the so-called "through spindle coolant method". In this method, coolant is fed from a coolant supply source to a rear end of a spindle and passes through passages formed on the spindle, a tool holder and a cutting tool in sequence. The coolant finally jets out from the free end of the cutting tool. Another method is the so-called "through tool coolant method". In this method, coolant is fed from a coolant supply source to a tool holder, other than to the rear end of the spindle, by a rotary joint provided on the tool holder and passes through the passages formed on the tool holder and the cutting tool attached to the tool holder in sequence and finally jets out from the free end of the cutting tool.
On the other hand, there are known rotating cutting tools, such as end mills, ball-end mills or drills which have a through passages formed in the cutting tools from a rear end face to a free end thereof along an axis of the cutting tools.
The first prior art is a method of, and an apparatus for, cutting work with a rotating cutting tool having a through passage or passages and being held on a tool holder. The through spindle coolant method or the through tool coolant method is employed.
The second prior art is disclosed in the Unexamined Japanese Utility Model Publication No. 1-132327, in which a tap to form holes for separating a molding has a through passage for cutting oil formed in the tap along its axis and channels for cutting oil extended on an outer surface of a shank of the tap in a direction parallel to the axis. In the case of tapping a blind hole, coolant fed through the passage and the channels will effect lubrication, cooling, and removal of the chips. On the other hand, in the case of the through hole, the coolant fed via the channels only effects lubrication, cooling, and removal of the chips, but the coolant fed through the passage will flow out from the workpiece without any effects.
The third prior art is disclosed in the Examined Japanese Utility Model Publication No. 4-2743, in which an end mill has at least one oil channel on an outer surface thereof. The channel is extended in a direction parallel to an axis of the end mill from a rear end to a chip removal flute thereof. Coolant, such as compressed air or cutting oil jets out through the oil channel, whereby the chips are removed smoothly. Furthermore, the end mill can be reground since there are no passages at its free end.
The fourth prior art is a method, and an apparatus disclosed in the Unexamined Japanese Patent Publication No. 4-25309, in which cutting edges with an ultra-fine grain are used and high-pressure water having a pressure of at least 10 kg/cm.sup.2 jets out from coolant discharge nozzles toward the cutting point. The nozzles are arranged separately from the cutting tool. The high-pressure water jet cools the cutting point immediately. The workpiece and the cutting tool are not damaged by heat since there is substantially no heat transfer to the cutting tool.
The fifth prior art is disclosed in the Unexamined Japanese Utility Model Publication No. 4-3440, in which coolant is supplied through coolant passages formed in a member rotating with a cutting tool. Discharge nozzles in the passages have adjustable jet angles so that the coolant can be applied to cutting edges of a cutting tool. The coolant is sufficiently applied to the cutting edges without being obstructed by the cutting tool since the coolant discharge nozzles rotate with the cutting tool. Furthermore, the relative position between the discharge nozzles and the cutting tool does not change when the cutting tool is fed in any direction along the X-, Y- and Z-axes, whereby the coolant is always sufficiently applied to the predetermined point.
The first prior art, mentioned above, is directed onto supply the coolant as possible as near the cutting point. To that end, through passages for the coolant are formed in the cutting tool. In the case of the cutting tool of carbide, however, it is difficult to form the passages. Also, in the case of the cutting tool having cutting edges near the center portion of the free end of the cutting tool, such as a ball-end mill or a drill, it is more difficult to form the passage. Because the passage must be made to bend or fork so that the exit passage is formed in the flank in order that it does not interfere with the action of the central portion of the free end of the cutting tool. Furthermore, in the case of a cutting tool having a small diameter, the through passage reduces the rigidity of the cutting tool. This results in the cutting tool tending to chip. Also, there is a problem in that insufficient coolant is supplied because of the small sectional area of the passage.
The second prior art discloses a tap as a tool cutting a thread used at relatively low cutting speed. Therefore, the coolant is preferably supplied to the cutting point at low pressure and the flow rate adjusted to the rotation of the tap so that the coolant flows into the threaded portion at very low rate. On the other hand, a cutting tool such as an end mill, a milling cutter or a drill is used at relatively high cutting speed in which the coolant must be supplied at high pressure and high flow rate. Therefore, the second prior art can not be applied to cutting work with such cutting tools.
In the third prior art, there is a problem that the coolant supplied at high pressure and flow rate will be interrupted by the cutting edges nearest to the shank since the coolant channels are extended only to the chip removal flute nearest the shank. Therefore, insufficient coolant will be applied to the main cutting part. In the case that the cutting tool is used at only its main cutting part, for example in the case of standard grooving work, an end mill is used at only its free end portion and the above problem becomes insurmountable. The insufficient cooling does not remove the chips quickly at the cutting point.
In the fourth prior art, the direction of the discharge nozzles of the coolant passages must be adjusted depending on the diameter and the length of the cutting tool used, so that the coolant is directed onto the cutting point since the direction of the nozzles are not arranged parallel to the axis of the cutting tool. Furthermore, there is another problem that the workpiece may interrupt the coolant applied to the cutting point directly depending on the shape of the workpiece.
In the fifth prior art, the exit of the coolant passage in the rotating member must face the outer surface of the cutting tool. Therefore, the jet angle of the exit of the coolant passages must be varied depending on the diameter and the length of the cutting tool used.
On the other hand, in practice, an end mill or a ball-end mill is not provided with cutting edges at the center of the free end thereof. Therefore, a single end mill or a single ball-end mill can not machine a pocket in a solid material by itself since large amount of heat is generated at the center region and the cutting tool is damaged by the heat. For example, in the prior art, when a die is machined from a solid material, firstly the solid material is bored by a drill then the prepared hole is made bigger by an end mill or a ball-end mill so that the workpiece is machined into the desired shape.
Thus, in this case, two types of cutting tools, that is, a drill and an end mill or a ball-end mill must be used and which must be changed during the machining process. Furthermore, it is necessary to provide two NC programs for the respective cutting tools. These problems lead to reduced efficiency and increased time and cost for the machining. There is, however, no successful method established to machine a pocket from a solid material by a single end mill or a single ball-end mill which is used for the rough cutting through the finishing.